Broody hen questions

Ok I actually have 2 different questions but they have to do with the same situation. lol

First off, I had a broody mama sitting on 7 eggs. I went away for a few days and wasn't around to collect eggs from the other chickens. So when I checked under her, there was a new egg. I candled it and sure enough, she'd been on it long enough to start growth. :( I couldn't bring myself to kill it so now it's under there with her. Only problem is, the others are due to hatch on Sunday (the 4th) and this one won't be due for at least a week after that. Is it at all possible that she will continue to sit until it hatches? I would assume not...that she'd just assume it wasn't growing and go about taking care of the new babies.

Also, my local hardware store is getting their chicks in on the 10th. That's 6 days after my scheduled hatch. Is there any way mama might accept some new chicks that late? I'm really not set up to brood them myself, as my hens have always done that for me. But I think only 3 of the 7 we set are going to hatch, and I'd really like a few more. Has anybody done that before - added new chicks to a mama within a week of her own being born?

Thanks! I went ahead and got 6 chicks and with any luck they'll be in on the 9th. It actually works out well because the ones hatching now are bantams and they're just so small...I'm glad they'll have a week to get bigger before we add the normal chicks. And my sebrights hatched a day early, so we have 2 beautiful silver chicks now. :)  I'm going to have to figure out what to do about the extra egg...